1. Field of the Invention
The present invention relates to a radar system that detects a target by using radio waves.
2. Description of the Related Art
A frequency-modulating continuous-wave (FM-CW) radar system has been developed as an in-vehicle radar system which is provided in a moving body such as a vehicle and which detects another vehicle, a person, an obstacle, or other object.
The FM-CW radar system transmits a frequency-modulated CW signal as a transmitting signal. It generates a beat signal by mixing the transmitting signal and a reception signal generated such that the transmitting signal is reflected by a target. It detects a relative position to the target and its relative speed based on the frequency of the beat signal.
A voltage-controlled oscillator (VCO) in which an oscillating frequency varies with a control voltage is used as a circuit for generating the transmitting signal. The relationship between the control voltage and oscillating frequency of the VCO is known beforehand. Thus, by controlling the control voltage supplied to the VCO, desired frequency modulation can be performed.
The characteristics of the oscillating frequencies of VCOs to the control voltages thereof are not always identical but have differences.
Regarding the above point, there are the following three related documents. (1) Japanese Unexamined Patent Application Publication No. 8-304532 discloses that the period of a triangular-waveform control voltage supplied to a VCO is divided into n intervals and two intervals among them are sequentially selected for triangular waveform correction. (2) Japanese Unexamined Patent Application Publication No. 11-271428 discloses that correcting data is obtained by comparing frequency spectra in n divisional intervals. (3) Japanese Unexamined Patent Application Publication No. 2001-174548 discloses that frequency modulation characteristics are measured by performing fast Fourier transform (FFT) processing in the portion of a modulation interval, extracting beat frequencies in a plurality of intervals, and transforming nonlinear distortion in numerical form based on the beat frequencies.
A radar system in any of the above-described documents (1), (2), and (3) uses a modulation method different from that used in an actual detection operation because correction is performed by using n divisional modulation intervals. Thus, it is impossible to perform correction in a form in which, while an actual detection operation is being performed, correcting data is simultaneously acquired.
Changes in characteristics of radar-system components, in particular, time-change characteristics (proportional to time or years) of a frequency-modulating voltage signal, have not been taken into consideration. Accordingly, it is impossible to cope with changes in characteristics occurring after shipping the radar system from a factory or after installing the radar system in an object such as a vehicle. In addition, in connection with the changes with time, malfunctions or other problems with the radar-system components cannot be detected.
In order to overcome the problems described above, preferred embodiments of the present invention provide a radar system in which time-change characteristics of a frequency-modulating voltage signal can be adjusted not only at the time of shipment from a factory, but also after shipment, that is, in an actual state of use.
According to a preferred embodiment of the present invention, a radar system includes a transmitting/receiving unit for transmitting, by supplying a frequency-modulating voltage signal to a voltage-controlled oscillator for determining a transmitting frequency, a transmitting signal in which an up-modulating interval having an upwardly shifting frequency and a down-modulating interval having a downwardly shifting frequency are alternately repeated, and for receiving a reflected signal from a target; a frequency-analyzing unit for finding data concerning the frequency spectrum of a beat signal based on the transmitting signal and the received signal; a detected-data extracting unit for extracting, based on the beat signal in the up-modulating interval and the beat signal in the down-modulating interval, at least one of a relative distance to the target and a relative speed of the target; and a time-change-characteristic adjustment unit for finding an evaluation value for evaluating the linearity of a frequency change in the transmitting signal from the result of the analysis by the frequency-analyzing unit, with a time-change characteristic of the frequency-modulating voltage signal changed by a minute amount, and for adjusting the time-change characteristic so that the evaluation value is optimal.
Preferably, the radar system of preferred embodiments of the present invention may include a warning output unit for outputting a warning when the amount for optimizing the evaluation value of adjusting the time-change characteristic of the frequency-modulating voltage signal exceeds a predetermined value.
In the radar system of preferred embodiments of the present invention, the time-change characteristic of the frequency-modulating voltage signal may be expressed by a polynomial of the second or higher order, and the time-change characteristic of the frequency-modulating voltage signal may be adjusted by changing the coefficient of each term of the polynomial.
According to preferred embodiments of the present invention, when adjustment is performed, a time-change characteristic of a frequency-modulating voltage signal is adjusted so that an evaluation value obtained based on the result of a frequency analysis by changing the time-change characteristic by a minute amount is optimal. Thus, the adjustment can be performed in a normal target-detecting operation.
The time-change characteristic of the frequency-modulating voltage signal is adjusted based on a beat signal caused by reflection from the target as one to be detected. Thus, in a factory, etc., it is not necessary to dispose a special target for characteristic adjustment or to connect an apparatus for characteristic adjustment. Accordingly, the adjustment can be performed in an actual state of use.
According to preferred embodiments of the present invention, in optimizing an evaluation value, when the amount of adjusting a time-change characteristic of a frequency-modulating voltage signal exceeds a predetermined value, data representing the state can be output. Therefore, a time change (year change) of a VCO can be detected, thus facilitating maintenance.
According to preferred embodiments of the present invention, a time-change characteristic of a frequency-modulating voltage signal is expressed by a polynomial of the second or higher order, and the time-change characteristic of the frequency-modulating voltage signal can be adjusted by changing the coefficient of each term. Thus, a smaller number of parameters can represent the time-change characteristic of the frequency-modulating voltage signal, and enables size and cost reduction of the entire apparatus without increasing the required memory capacity.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.